The primary git repository has been moved from
www.swi-prolog.org to
github.com. That means that developers
need to push to the github repository. The repos at www.swi-prolog.org
are synchronized three times per day. What does this mean?

Update an existing working tree to GitHub

Use the commands below to update a working tree for the development branch.
Use swipl.git for the stable series. If you have a GitHub account
and setup ssh, you can also use
github@github.com:SWI-Prolog/swipl-devel.git.

Checkout old versions

The submodule locations have changed on April 22, 2014. If you want to
check out an older version, use the repositories at www.swi-prolog.org.
You can use the above commands to switch freely between both sources or
use git remote add to add both of them.